Marine and terrestrial vertebrates are used to examine the role of organic ion transport in the renal and hepatic excretion of environmental contaminants such as DDT and 2,4-dichlorophenoxyacetic acid (2,4-D). Research focuses on the contribution of such transport to 1) the rate of xenobiotic depurition, 2) its toxicity to the organism, and 3) the interference of the foreign compound with elimination of endogenous wastes or toxins. The role of transport in the elimination of xenobiotics from specific organs, e.g., brain, as well as from the whole organism are also characterized. BIBLIOGRAPHIC REFERENCES: Pritchard, J. B.: In vitro analysis of DDA handling by rat kidney and liver. Toxicol. Appl. Pharmacol. 38: 621-630, 1976.